Technical Report Documentation Page 1. Report No. 2. Gov. 3. Recipient's Catalog No. FHWA-NH-RD-14282F Accession No. 4. Title and Subtitle 5. Report Date Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New 2008 Hampshire 7. Author(s) 8. Performing Organization Report No. Scott A. Olson 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) U. S. Geological Survey New Hampshire-Vermont Water Science Center 361 Commerce Way Pembroke, NH 03275 11. Contract or Grant No. 14282F, X-A000 (601) 12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered New Hampshire Department of Transportation FINAL REPORT 7 Hazen Drive, PO Box 483 Concord, NH 03302-0483 14. Sponsoring Agency Code 15. Supplementary Notes In cooperation with the U. S. Department of Transportation, Federal Highway Administration 16. Abstract This report provides estimates of flood discharges at selected recurrence intervals for streamgages in and adjacent to New Hampshire and equations for estimating flood discharges at recurrence intervals of 2-, 5-, 10-, 25-, 50-, 100-, and 500-years for ungaged, unregulated, rural streams in New Hampshire. The equations were developed using generalized least-squares regression. Flood-frequency and drainage-basin characteristics from 117 streamgages were used in developing the equations. The drainage-basin characteristics used as explanatory variables in the regression equations include drainage area, mean April precipitation, percentage of wetland area, and main channel slope. The average standard error of prediction for estimating the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year recurrence interval flood discharges with these equations are 30.0, 30.8, 32.0, 34.2, 36.0, 38.1, and 43.4 percent, respectively. Flood discharges at selected recurrence intervals for selected streamgages were computed following the guidelines in Bulletin 17B of the U.S. Interagency Advisory Committee on Water Data. To determine the flood-discharge exceedence probabilities at streamgages in New Hampshire, a new generalized skew coefficient map covering the State was developed. The standard error of the data on the new map is 0.298. To improve estimates of flood discharges at selected recurrence intervals for 20 streamgages with short-term records (10 to 15 years), record extension using the two-station comparison technique was applied. The two-station comparison method uses data from a streamgage with long-term record to adjust the frequency characteristics at a streamgage with a short-term record. A technique for adjusting a flood-discharge frequency curve computed from a streamgage record with results from the regression equations is described in this report. Also, a technique is described for estimating flood discharge at a selected recurrence interval for an ungaged site upstream or downstream from a streamgage using a drainage-area adjustment. The final regression equations and the flood-discharge frequency data used in this study will be available in StreamStats. StreamStats is a World Wide Web application providing automated regression-equation solutions for user-selected sites on streams. 17. Key Words 18. Distribution Statement Streamflow, Stream gages, mathematical prediction, Scour, Design flood, Design storm, Peak No restrictions. This document is available discharge, Bridge design, Geographic information systems, Regression analysis, Data to the public through the National Technical collection, New Hampshire Information Service, Springfield, Virginia, 22161 19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price Unclassified Unclassified 64 DISCLAIMER This document is disseminated under the sponsorship of the New Hampshire Department of Transportation (NHDOT) and the U.S. Department of Transportation Federal Highway Administration (FHWA) in the interest of information exchange. The NHDOT and FHWA assume no liability for the use of information contained in this report. The document does not constitute a standard, specification, or regulation. The NHDOT and FHWA do not endorse products, manufacturers, engineering firms, or software. Products, manufacturers, engineering firms, software, or proprietary trade names appearing in this report are included only because they are considered essential to the objectives of the document. Prepared in cooperation with the New Hampshire Department of Transportation Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire Scientific Investigations Report 2008–5206 U.S. Department of the Interior U.S. Geological Survey Cover. Photograph of Suncook River at Main Street in the Village of Suncook, New Hampshire, during the Flood of May 2006. Estimation of Flood Discharges at Selected Recurrence Intervals for Streams in New Hampshire By Scott A. Olson Prepared in cooperation with the New Hampshire Department of Transportation Scientific Investigations Report 2008–5206 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DIRK KEMPTHORNE, Secretary U.S. Geological Survey Mark D. Myers, Director U.S. Geological Survey, Reston, Virginia: 2009 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Olson, S.A., 2009, Estimation of flood discharges at selected recurrence intervals for streams in New Hampshire: U.S. Geological Survey Scientific Investigations Report 2008–5206, 57 p. iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Purpose and Scope ..............................................................................................................................2 Description of Study Area ...................................................................................................................2 Flood Discharges at Selected Recurrence Intervals for Streamgages ................................................4 Peak Discharge Data Used in this Study ..........................................................................................4 Determination of the Magnitude and Frequency of Flood Discharges for Streamgages .........7 Generalized Skew ........................................................................................................................7 Record Extension .........................................................................................................................7 Combined Records of Nearby Streamgages ...........................................................................7 Magnitude and Frequency of Flood Discharges for Streamgages ............................................10 Maximum Recorded Floods and Envelope Curves ........................................................................10 Characteristics of Streamgage Drainage Basins ...................................................................................10 Regression Equations for Estimation of Flood Discharges at Selected Recurrence Intervals for Ungaged Stream Sites .............................................................................................................13 Regression Analysis and Final Regression Equations ..................................................................13 Limitations and Sensitivity .................................................................................................................14 Accuracy of the Regression Equations ...........................................................................................16 Accuracy Analysis of Individual Estimates from the Regression Equations ............................17 Standard Error of Prediction ....................................................................................................17 Equivalent Years of Record ......................................................................................................19 Prediction Intervals ...................................................................................................................19 Use of Regression Equations at or Near Streamgages ................................................................19 Drainage-Area-Only Regression Equations ...................................................................................20 New Hampshire StreamStats ....................................................................................................................22 Summary........................................................................................................................................................22 Selected References ...................................................................................................................................22